I couldn't find anything "concrete" and we do not allow assumptions, therefore the following is only opinion.
I will assume you have a written agreement between you and the person assuming the loan, otherwise the transaction is not reportable. The person assuming the loan presented an application to do so, you approved the assumption and the rate was set for that person. IMO, you will use the date the final rate was set for the person assuming the loan. That rate could be the loan's current rate or a different rate but the rate was set sometime during the application process for the person assuming the loan.
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The opinions expressed are mine and they are not to be taken as legal advice.